1. Field of the Invention
The present invention relates to a semiconductor device using a III-V group compound semiconductor and particularly, a nitride semiconductor containing gallium which is expressed by a general formula of In1-x-yAlxGayN (0 y 1 and 0 x+y 1), and a method of fabricating the same.
2. Description of the Background Art
It has been expected that a semiconductor device using a nitride semiconductor such as GaN, InGaN, AlGaN, or AlGaInN is applied to a light receiving element and a light emitting element for receiving and emitting light in a region from a visible region to a ultraviolet region, and an environmental resistance electronic device used under high temperatures, a high-frequency and high-power electronic device used for mobile communication, or the like.
In the above-mentioned semiconductor device using the nitride semiconductor, a nitride-semiconductor layer is formed on a substrate composed of sapphire, spinel, Si, SiC, GaP, GaAs, or the like using MOVPE (Metal Organic Vapor Phase Epitaxy), MBE (Molecular Beam Epitaxy), HVPE (Halide Vapor Phase Epitaxy), or the like. The difference between the lattice constant of the substrate and the lattice constant of the nitride-semiconductor layer is large. If the nitride-semiconductor layer is directly formed on the substrate, therefore, it is difficult for the nitride-semiconductor layer to have good crystalline quality. In order to solve the problem caused by the difference in the lattice constant, the nitride-semiconductor layer is formed on the substrate through a buffer layer composed of AlN or GaN in the conventional semiconductor device, as disclosed in JP-A-2-81482 and JP-A-8-64868.
FIG. 10 is a schematic sectional view showing the construction of a light emitting diode which is an example of the above-mentioned conventional semiconductor device using the nitride semiconductor.
In the light emitting diode shown in FIG. 10, a buffer layer 102 composed of AlN or GaN, an n-type contact layer 103 composed of n-type GaN, an n-type cladding layer 104 composed of n-type AlGaN, a light-emitting layer 105 composed of GaInN, a p-type cladding layer 106 composed of p-type AlGaN, and a p-type contact layer 107 composed of p-type GaN are successively formed on a substrate 101 composed of sapphire, spinel, Si, SiC, GaP, GaAs, or the like. A device-constituting layer 120 constituting a device portion of the light emitting diode comprises the n-type contact layer 103, the n-type cladding layer 104, the light-emitting layer 105, the p-type cladding layer 106, and the p-type contact layer 107.
A p-side electrode 108 having transparency is formed on the p-type contact layer 107, a pad electrode 109 is formed thereon, and an n-side electrode 110 is formed on the n-type contact layer 103.
As described above, in the conventional light emitting diode, the crystalline quality of the device-constituting layer 120 is made better by forming the device-constituting layer 120 on the substrate 101 through the buffer layer 102, thereby improving the luminous characteristics of the light emitting diode.
In the conventional light emitting diode, however, the following problems arise because the buffer layer 102 is formed at a lower temperature than a single-crystal-growth-temperature.
First, the buffer layer 102 formed at such a lower temperature has a lot of defects such as unbonding joints or grain boundaries because it is in an amorphous or polycrystalline state. Since the defects are propagated to the device-constituting layer 120 at the time of forming the device-constituting layer 120, therefore, it is impossible for the device-constituting layer 120 to have good crystalline quality.
When the device-constituting layer 120 is formed on the buffer layer 102 formed at the low temperature, In (indium) atoms or Ga (gallium) atoms are easily concentrated on a particular portion upon being diffused through a crystal growth plane of the buffer layer 102 in the early stages of formation. In the early stages of formation, therefore, the device-constituting layer 120 is grown as crystals in an island shape around the particular portion. Accordingly, a lot of defects such as grain boundaries or nano-pipes occur, thereby degrading the crystalline quality of the device-constituting layer 120.
Furthermore, a nitride containing Ga or In, for example, GaN or InN generally has the property of easy desorption of N (nitrogen). When the device-constituting layer 120 is formed on the buffer layer 102 formed at the low temperature, therefore, nitrogen is desorbed particularly from the microcrystals grown in an island shape in the vicinity of the interface of the device-constituting layer 120 and the buffer layer 102, thereby causing a new defect. As a result, the defect is propagated through the device-constituting layer 120, to reach the top thereof, thereby degrading the crystalline quality of the entire device-constituting layer 120.